MOLECULAR DENSITY-FUNCTIONAL CALCULATIONS IN THE REGULAR RELATIVISTICAPPROXIMATION - METHOD, APPLICATION TO COINAGE METAL DIATOMICS, HYDRIDES, FLUORIDES AND CHLORIDES, AND COMPARISON WITH FIRST-ORDER RELATIVISTIC CALCULATIONS

The application of the zeroth-order regular relativistic approximation (ZORA) for molecular density functional calculations is investigated. By introducing a model potential to construct the kinetic energy oper ator, stationarity of the energy with respect to orbital variations is gained and most problems connected with gauge dependence of the regul ar approximation are eliminated. The formulation of a geometry gradien t is greatly facilitated using this formalism. Calculations for the co inage metal hydrides (Cu-2, Ag-2, Au-2) as well as for the homonuclear (Cu-2, Ag-2, Au-2 and heteronuclear (CuAg, CuAu, AgAu) diatomics show that the results of ZORA calculations within the electrostatic shift approximation, as introduced by van Lenthe and co-workers, can be dupl icated using the simpler scheme proposed in this work. Results for the coinage metal fluorides (CuF, AgF, AuF) and chlorides (CuCl, AgC1, Au Cl) are presented as well. First-order relativistic calculations have been performed for all systems to assess the applicability of leading- order relativistic perturbation theory. (C) 1998 American Institute of Physics.